Fc fusion proteins of human erythropoietin with high biological activities

ABSTRACT

Fc fusion proteins of human EPO with high biological activities relative to rHuEPO on a molar basis are disclosed. The HuEPO-L-vFc fusion protein comprises HuEPO, a flexible peptide linker of about 20 or fewer amino acids, and a human IgG Fc variant. The Fc variant is of a non-lytic nature and shows minimal undesirable Fc-mediated side effects. A method is also disclosed to make or produce such fusion proteins at high expression levels. Such HuEPO-L-vFc fusion proteins exhibit extended serum half-life and increased biological activities, leading to improved pharmacokinetics and pharmacodynamics, thus fewer injections will be needed within a period of time.

BACKGROUND

[0001] Erythropoietin (EPO) is a 30.4 kilodalton (kDa) glycoprotein hormone that promotes the proliferation of erythroid progenitor cells and supports their differentiation into mature erythrocytes (see, for example, Krantz, Blood, 77:419-434, 1991). EPO is produced in the adult kidney and the fetal liver. In adults, EPO is produced primarily in kidney cells in response to hypoxia or anemia and circulates in the bloodstream. EPO targets the 66 kDa specific receptor (EPO-Rc) found almost exclusively on the surface of erythroid progenitor cells present in bone marrow. Upon binding EPO, the receptor is activated and undergoes homodimerization, followed by tyrosine phosphorylation. Subsequently, a series of intracellular signal transduction events take place, leading to the increase of the number of the progenitor cells and their maturation into erythrocytes (see, for example, Lodish et al., Cold Spring Harbor Symp. Quant. Biol., 60:93-104, 1995).

[0002] Recombinant human EPO (rHuEPO) is widely used in the treatment of patients with chronic anemia due to renal diseases at both end-stage and pre-dialysis phases. Administration of EPO has also been successful to treat anemia in patients caused by cancer chemotherapy, rheumatoid arthritis, AZT treatment for HIV infection and myelodysplastic syndrome. No direct toxic effect of treatment has been reported and the benefits of blood transfusion could be achieved without the transfusion.

[0003] The concentration of EPO in normal human serum varies approximately from 0.01 to 0.03 units/ml. Supplemental EPO is a desirable treatment in cases of renal failure with decreased EPO production. The half-life for the serum clearance of intravenous (i.v.) rHuEPO is approximately 4 to 13 h. The peak serum concentration for subcutaneous (s.c.) rHuEPO occurs in 5 to 24 h after injection with an elimination half-life of 17 h. The s.c. administration route can therefore lead to much longer retention in the blood than i.v. administration of the same dose. The mechanism responsible for clearing EPO from the serum remains unclear. In animal experiments, less than 5% is excreted by the kidney. The liver, which rapidly removes asialated EPO, has not been shown to play a significant role in clearing EPO (see, for example, Fried, Annu. Rev. Nutr., 15:353-377, 1995).

[0004] Immunoglobulins of IgG class are among the most abundant proteins in human blood. Their circulation half-lives can reach as long as 21 days. Fusion proteins have been reported to combine the Fc regions of IgG with the domains of another protein, such as various cytokines and soluble receptors (see, for example, Capon et al., Nature, 337:525-531, 1989; Chamow et al., Trends Biotechnol., 14:52-60, 1996); U.S. Pat. Nos. 5,116,964 and 5,541,087). The prototype fusion protein is a homodimeric protein linked through cysteine residues in the hinge region of IgG Fc, resulting in a molecule similar to an IgG molecule without the CH1 domains and light chains. Due to the structural homology, Fc fusion proteins exhibit in vivo pharmacokinetic profile comparable to that of human IgG with a similar isotype. This approach has been applied to several therapeutically important cytokines, such as IL-2 and IFN-α_(2a), and soluble receptors, such as TNF-Rc and IL-5-Rc (see, for example, U.S. Pat. Nos. 5,349,053 and 6,224,867). To extend the circulating half-life of EPO and/or to increase its biological activity, it is desirable to make fusion proteins containing EPO linked to the Fc portion of the human IgG protein as disclosed or described in this invention.

[0005] In most of the reported Fc fusion protein molecules, a hinge region serves as a spacer between the Fc region and the cytokine or soluble receptor at the amino-terminus, allowing these two parts of the molecule to function separately (see, for example, Ashkenazi et al., Current Opinion in Immunology, 9:195-200, 1997). Relative to the EPO monomer, a fusion protein consisting of two complete EPO domains separated by a 3- to 7-amino acid peptide linker exhibited reduced activity (Qiu et al., J. Biol. Chem., 273:11173-11176, 1998). However, when the peptide linker between the two EPO domains was 17 amino acids in length, the dimeric EPO molecule exhibited considerably enhanced in vitro and in vivo activities. The enhanced activity has been shown to be due to an increased in vitro activity coupled with a different pharmacokinetic profile in mice (see, for example, Sytkowski et al., J. Biol. Chem., 274:24773-24778, 1999; U.S. Pat. No. 6,187,564 ). It is discovered according to this invention that an added peptide linker present between HUEPO and a human IgG Fc variant renders the in vitro biological activity of the HuEPO-L-Fc molecule high. A human EPO fusion protein with an appropriate peptide linker between the HuEPO and Fc moieties (HuEPO-L-Fc) exhibits an in vitro biological activity similar to or higher than that of rHuEPO itself on a molar basis. While not bound by any theory, it is believed that this “similar to or higher than” in vitro biological activity is achieved in two ways: (1) keeping the Fc region away from the EPO-Rc binding sites on EPO, and (2) keeping one EPO from the other EPO domain, so both EPO domains can interact with EPO-Rc on the erythroid progenitor cells independently. For the present invention, a flexible peptide linker of about 20 or fewer amino acids in length is preferred. It is preferably to use a peptide linker comprising of two or more of the following amino acids: glycine, serine, alanine, and threonine.

[0006] The Fc region of human immunoglobulins plays a significant role in immune defense for the elimination of pathogens. Effector functions of IgG are mediated by the Fc region through two major mechanisms: (1) binding to the cell surface Fc receptors (Fc_(γ)Rs) can lead to ingestion of pathogens by phagocytosis or lysis by killer cells via the antibody-dependent cellular cytotoxicity (ADCC) pathway, or (2) binding to the C1q part of the first complement component C1 initiates the complement-dependent cytotoxicity (CDC) pathway, resulting in the lysis of pathogens. Among the four human IgG isotypes, IgG1 and IgG3 are effective in binding to Fc_(γ)R. The binding affinity of IgG4 to Fc_(γ)R is an order of magnitude lower than that of IgG1 or IgG3, while binding of IgG2 to Fc_(γ)R is below detection. Human IgG1 and IgG3 are also effective in binding to C1q and activating the complement cascade. Human IgG2 fixes complement poorly, and IgG4 appears quite deficient in the ability to activate the complement cascade (see, for example, Jefferis et al., Immunol. Rev., 163:59-76, 1998). For therapeutic use in humans, it is essential that when HuEPO-L-Fc binds to EPO-Rc on the surface of the erythroid progenitor cells, the Fc region of the fusion protein will not mediate undesirable effector functions, leading to the lysis or removal of these progenitor cells. Accordingly, the Fc region of HuEPO-L-Fc must be of a non-lytic nature, i. e. the Fc region must be inert in terms of binding to Fc_(γ)Rs and C1q for the triggering of effector functions. It is clear that none of the naturally occurring IgG isotypes is suitable for use to produce the HuEPO-L-Fc fusion protein. To obtain a non-lytic Fc, certain amino acids of the natural Fc region have to be mutated for the attenuation of the effector functions.

[0007] By comparing amino acid sequences of human and murine IgG isotypes, a portion of Fc near the N-terminal end of the CH2 domain is implicated to play a role in the binding of IgG Fc to Fc_(γ)Rs. The importance of a motif at positions 234 to 237 has been demonstrated using genetically engineered antibodies (see, for example, Duncan et al., Nature, 332:563-564, 1988). The numbering of the amino acid residues is according to the EU index as described in Kabat et al. (in Sequences of Proteins of Immunological Interest, 5^(th) Edition, United States Department of Health and Human Services, 1991). Among the four human IgG isotypes, IgG1 and IgG3 bind Fc_(γ)Rs the best and share the sequence Leu234-Leu-Gly-Gly237 (only IgG1 is shown in FIG. 1). In IgG4, which binds Fc_(γ)Rs with a lower affinity, this sequence contains a single amino acid substitution, Phe for Leu at position 234. In IgG2, which does not bind Fc_(γ)Rs, there are two substitutions and a deletion leading to Val234-Ala-Gly237 (FIG. 1). To minimize the binding of Fc to Fc_(γ)R and hence the ADCC activity, Leu235 in IgG4 has been replaced by Ala (see, for example, Hutchins et al., Proc. Natl. Acad. Sci. USA, 92:11980-11984, 1995). IgG1 has been altered in this motif by replacing Glu233-Leu-Leu235 with Pro233-Val-Ala235, which is the sequence from IgG2. This substitution resulted in an IgG1 variant devoid of Fc_(γ)R-mediated ability to deplete target cells in mice (see, for example, Isaacs et al., J. Immunol., 161: 3862-3869, 1998).

[0008] A second portion that appears to be important for both Fc_(γ)R and C1q binding is located near the carboxyl-terminal end of CH2 domain of human IgG (see, for example, Duncan et al., Nature, 332:738-740, 1988). Among the four human IgG isotypes, there is only one site within this portion that shows substitutions: Ser330 and Ser331 in IgG4 replacing Ala330 and Pro331 present in IgG1, IgG2, and IgG3 (FIG. 1). The presence of Ser330 does not affect the binding to Fc_(γ)R or C1q. The replacement of Pro331 in IgG1 by Ser virtually abolished IgG1 ability to C1q binding, while the replacement of Ser331 by Pro partially restored the complement fixation activity of IgG4 (see, for example, Tao et al., J. Exp. Med. 178:661-667, 1993; Xu et al., J. Biol. Chem., 269:3469-3474, 1994).

[0009] We discover that at least three Fc variants (vFc) can be designed for the production of HuEPO-L-vFc fusion proteins (FIG. 1). Human IgG2 Fc does not bind Fc_(γ)R but showed weak complement activity. An Fc_(γ2) variant with Pro331Ser mutation should have less complement activity than natural Fc_(γ2) while remain as a non-binder to Fc_(γ)R. IgG4 Fc is deficient in activating the complement cascade, and its binding affinity to Fc_(γ)R is about an order of magnitude lower than that of the most active isotype, IgG1. An Fc_(γ4) variant with Leu235Ala mutation should exhibit minimal effector functions as compared to the natural Fc_(γ4). The Fc_(γ1) variant with Leu234Val, Leu235Ala and Pro331Ser mutations also will exhibit much less effector functions than the natural Fc_(γ1). These Fc variants are more suitable for the preparation of the EPO fusion proteins than naturally occurring human IgG Fc. It is possible that other replacements can be introduced for the preparation of a non-lytic Fc without compromising the circulating half-life or causing any undesirable conformational changes.

[0010] There are many advantages with the present invention. The high activity and prolonged presence of the HuEPO-L-vFc fusion protein in the serum can lead to lower dosages as well as less frequent injections. Less fluctuations of the drug in serum concentrations also means improved safety and tolerability. Less frequent injections may result in better patient compliance and quality of life. The HuEPO-L-vFc fusion protein containing a non-lytic Fc variant will therefore contribute significantly to the management of anemia caused by conditions including renal failure, cancer chemotherapy, rheumatoid arthritis, AZT treatment for HIV infection, and myelodysplastic syndrome.

SUMMARY OF THE INVENTION

[0011] One aspect of the present invention relates to a HuEPO-L-vFc fusion protein. The HuEPO-L-vFc fusion protein comprises HuEPO, a peptide linker, and a human IgG Fc variant. It is preferably to use a flexible peptide linker of 20 or fewer amino acids in length which comprises of two or more of the following amino acids: glycine, serine, alanine, and threonine. The IgG Fc variant is of non-lytic nature and contains amino acid mutations as compared to naturally occurring IgG Fc.

[0012] It is another embodiment of the present invention that the human Ig Fc comprises a hinge, CH2, and CH3 domains of human IgG, such as human IgB1, IgG2, and IgG4. The CH2 domain contains amino acid mutations at positions 228, 234, 235, and 331 (defined by the EU numbering system) to attenuate the effector functions of Fc.

[0013] In yet another embodiment of the present invention, a method is disclosed to make or produce such fusion proteins from a mammalian cell line such as a CHO-derived cell line. Growing transfected cell lines under conditions such that the recombinant fusion protein is expressed in its growth medium in excess of 10, preferably 30, μg per million cells in a 24 hour period. These HuEPO-L-vFc fusion proteins exhibit an in vitro biological activity similar to or higher than that of rHuEPO on a molar basis, and more preferably an extended serum half-life without undesirable side effects, leading to improved pharmacokinetics and pharmacodynamics, thus lower dosages and fewer injections would be needed to achieve similar efficacies.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0014]FIG. 1 shows the amino acid sequence alignment from the hinge and CH2 regions of human IgB1, IgG2, IgG4 and their variants. Three portions are compared: amino acid position 228, 234-237, and 330-331. Amino acid mutations of the variants are indicated in bold italics. The EU numbering system is used for the amino acid residues.

[0015]FIG. 2 shows the nucleotide sequence and deduced amino acid sequence of (A) HuEPO-L-vFc_(γ2), (B) HuEPO-L-vFc_(γ4), and (C) HuEPO-L-vFc_(γ1) as the HindIII-EcoRI fragment in the respective pEFP expression vector. The peptide from amino acid residues −27 to −1 is the leader peptide of human EPO. The mature protein contains human EPO (amino acid residues 1 to 165), a peptide linker (amino acid residues 166 to 181), and a Fc variant (amino acid residues 182 to 409 of vFc_(γ2), 182 to 410 of vFc_(γ4), and 182 to 408 of vFc_(γ1)). In the Fc regions, nucleotide and corresponding amino acid mutations in bold are also underlined.

[0016]FIG. 3 shows the effect of the purified proteins, HuEPO-L-vFc_(γ2) or rHuEPO, on the proliferation of the 32D1.9 cells.

[0017]FIG. 4 shows the effect of the purified HuEPO-L-vFc_(γ2) protein on the red blood cell counts in rats.

[0018]FIG. 5 shows the effect of the purified HuEPO-L-vFc_(γ2) protein on the hemaglobin values in rats.

[0019]FIG. 6 shows circulating levels of HuEPO-L-vFc_(γ2) in rats after i.v. injections of the fusion protein.

DETAILED DESCRIPTION OF THE INVENTION

[0020] 1. Construction of the Gene Encoding the Fusion Proteins

[0021] 1.1 Construction of the Gene Encoding the HuEPO-L-vFc_(γ2) Fusion Protein

[0022] A fusion protein is assembled from several DNA segments. To obtain the gene encoding the leader peptide and mature protein of human EPO, cDNA library of human fetal liver or kidney (obtained from Invitrogen, Carlsbad, Calif.) is used as the template in polymerase chain reaction (PCR). For the convenience of cloning, SEQ ID NO:1 (Table 1), which incorporates a restriction enzyme cleavage site (HindIII) is used as the 5′ oligonucleotide primer. Table 1 shows the sequences of oligonucleotides used for the cloning of the HuEPO-L-vFc fusion proteins. The 3′ primer (SEQ ID NO:2) eliminates the EPO termination codon and incorporates a BamHI site. The resulting DNA fragments of approximately 600 bp in length are inserted into a holding vector such as pUC19 at the HindIII and BamHI sites to give the pEPO plasmid. The sequence of the human EPO gene is confirmed by DNA sequencing. TABLE 1 Sequences of Oligonucleotides. SEQ ID NO: 1 5′-cccaagcttggcgcggagatgggggtgca-3′ SEQ ID NO: 2 5′-cggatccgtcccctgtcctgcaggcct-3′ SEQ ID NO: 3 5′-gagcgcaaatgttgtgtcga-3′ SEQ ID NO: 4 5′-ggaattctcatttacccggagacaggga-3′ SEQ ID NO: 5 5′-tggttttctcgatggaggctgggaggcct-3′ SEQ ID NO: 6 5′-aggcctcccagcctccatcgagaaaacca-3′ SEQ ID NO: 7 5′-cggatccggtggcggttccggtggaggcggaagcggcggtggaggat cagagcgcaaatgttgtgtcga-3′ SEQ ID NO: 8 5′-gagtccaaatatggtccccca-3′ SEQ ID NO: 9 5′-ggaattctcatttacccagagacaggga-3′ SEQ ID NO: 10 5′-cctgagttcgcggggggacca-3′ SEQ ID NO: 11 5′-gagtccaaatatggtcccccatgcccaccatgcccagcacctgagtt cgcggggggacca-3′ SEQ ID NO: 12 5′-cggatccggtggcggttccggtggaggcggaagcggcggtggaggat cagagtccaaatatggtccccca-3′ SEQ ID NO: 13 5′-gacaaaactcacacatgccca-3′ SEQ ID NO: 14 5′-acctgaagtcgcggggggaccgt-3′ SEQ ID NO: 15 5′-gacaaaactcacacatgcccaccgtgcccagcacctgaagtcgcggg gggaccgt-3′ SEQ ID NO: 16 5′-cggatccggtggcggttccggtggaggcggaagcggcggtggaggat cagacaaaactcacacatgccca-3′

[0023] The gene encoding the Fc region of human IgG2 (Fc_(γ2)) is obtained by reverse transcription and PCR using RNA prepared from human leukocytes and appropriate 5′ (SEQ ID NO:3) and 3′ (SEQ ID NO:4) primers. Resulting DNA fragments of Fc_(γ2) containing complete sequences of the hinge, CH2, and CH3 domains of IgG2 will be used as the template to generate the FC_(γ2) Pro331Ser variant (vFc_(γ2)) in which Pro at position 331 of Fc_(γ2) is replaced with Ser. To incorporate this mutation, two segments are produced and then assembled by using the natural Fc_(γ2) as the template in overlapping PCR. The 5′ segment is generated by using SEQ ID NO:3 as the 5′ primer and SEQ ID NO:5 as the 3′ primer. The 3′ segment is generated by using SEQ ID NO:6 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. These two segments are then joined at the region covering the Pro331Ser mutation by using SEQ ID NO:7 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. The SEQ ID NO:7 primer contains sequences encoding a 16-amino acid Gly-Ser peptide linker including a BamHI restriction enzyme site. The resulting DNA fragments of approximately 700 bp in length are inserted into a holding vector such as pUC19 at the BamHI and EcoRI sites to give the pL-vFcγ2 plasmid. The sequence of the gene is confirmed by DNA sequencing.

[0024] To prepare the HuEPO-L-vFc_(γ2) fusion gene, the EPO fragment is excised from the pEPO plasmid with HindlIl and BamHI and is purified by agarose gel electrophoresis. The purified fragment is then inserted to the 5′-end of the peptide linker in the pL-vFcγ2 plasmid to give the pEPO-L-vFcγ2 plasmid. The fusion gene comprises HuEPO, a Gly-Ser peptide linker and the Fc_(γ2) variant gene.

[0025] The presence of a peptide linker between the EPO and Fc moieties increases the flexibility of the EPO domains and enhances its biological activity (see, for example, Sytkowski et al., J. Biol. Chem., 274: 24773-8, 1999). For the present invention, a peptide linker of about 20 or fewer amino acids in length is preferred. Peptide linker comprising two or more of the following amino acids: glycine, serine, alanine, and threonine can be used. An example of the peptide linker contains Gly-Ser peptide building blocks, such as GlyGlyGlyGlySer. FIG. 2A shows a fusion gene (SEQ ID NO: 17) containing sequences encoding HUEPO, a 16-amino acid peptide linker (GlySerGlyGlyGlySerGlyGlyGlyGlySerGlyGlyGlyGlySer, SEQ ID NO: 23), and the Fc_(γ2) Pro331Ser variant, and its corresponding amino acid sequence (SEQ ID NO: 18).

[0026] The complete gene encoding the HuEPO-L-vFc fusion protein is then inserted at the HindIII and EcoRI sites of a mammalian expression vector, such as pcDNA3 (Invitrogen). The final expression vector plasmid, named pEFP2, contains the cytomegalovirus early gene promoter-enhancer which is required for high level expression in mammalian cells. The plasmid also contains selectable markers to confer ampicillin resistance in bacteria, and G418 resistance in mammalian cells. In addition, the pEFP2 expression vector contains the dihydrofolate reductase (DHFR) gene to enable the co-amplification of the HuEPO-L-vFc_(γ2) fusion gene and the DHFR gene in the presence of methotrexate (MTX) when the host cells are deficient in the DHFR gene expression (see, for example, U.S. Pat. No. 4,399,216).

[0027] 1.2 Construction of the Gene Encoding the HuEPO-L-vFc_(γ4) Fusion Protein

[0028] Human IgG4 is observed partly as half antibody molecules due to the dissociation of the inter-heavy chain disulfide bonds in the hinge domain. This is not seen in the other three human IgG isotypes. A single amino acid substitution replacing Ser228 with Pro, which is the residue found at this position in IgG1 and IgG2, leads to the formation of IgG4 complete antibody molecules (see, for example, Angal et al., Molec. Immunol., 30:105-108, 1993; Owens et al., Immunotechnology, 3:107-116, 1997; U.S. Pat. No. 6,204,007). The Fc_(γ4) variant containing Leu235Ala mutation for the minimization of FcR binding will also give rise to a homogeneous fusion protein preparation with this additional Ser228Pro mutation.

[0029] The gene encoding the Fc region of human IgG4 (Fc_(γ4)) is obtained by reverse transcription and PCR using RNA prepared from human leukocytes and appropriate 5′ primer (SEQ ID NO:8) and 3′ primer (SEQ ID NO:9). Resulting DNA fragments of Fc_(γ)4 containing complete sequences of the hinge, CH2, and CH3 domains of IgG4 is used as the template to generate the Fc_(γ4) variant with Ser228Pro and Leu235Ala mutations (vFc_(γ4)) in which Ser228 and Leu235 have been replaced with Pro and Ala, respectively. The CH2 and CH3 domains are amplified using the 3′ primer (SEQ ID NO:9) and a 5′ primer containing the Leu235Ala mutation (SEQ ID NO:10). This amplified fragment, together with a synthetic oligonucleotide of 60 bases in length (SED ID NO:11) containing both Ser228Pro and Leu235Ala mutations, are joined in PCR by using SEQ ID NO:12 as the 5′ primer and SEQ ID NO:9 as the 3′ primer. The SEQ ID NO:12 primer contains sequences encoding a 16-amino acid Gly-Ser peptide linker including the BamHI site. The resulting DNA fragments of approximately 700 bp in length are inserted into a holding vector such as pUC19 at the BamHI and EcoRI sites to give the pL-vFcγ4 plasmid. The sequence of the gene is confirmed by DNA sequencing.

[0030] To prepare the HuEPO-L-vFC_(γ4) fusion gene, the HuEPO fragment is excised from the pEPO plasmid with HindIII and BamHI and then inserted to the 5′-end of the peptide linker in the pL-vFcγ4 plasmid to give the pEPO-L-vFcγ4 plasmid. This fusion gene comprising HuEPO, a 16-amino acid Gly-Ser peptide linker and the Fc_(γ4) variant gene is then inserted at the HindIII and EcoRI sites of a mammalian expression vector, such as pcDNA3 (Invitrogen), as described for the HuEPO-L-vFC_(γ2) fusion protein. The final expression vector plasmid is designated as pEFP4. FIG. 2B shows a fusion gene (SEQ ID NO:19) containing sequences encoding HuEPO, a 16-amino acid peptide linker (GlySerGlyGlyGlySerGlyGlyGlyGlySerGlyGlyGlyGlySer, SEQ ID NO:23), and the Fc_(γ4) variant with Ser228Pro and Leu235Ala mutations, and its corresponding amino acid sequence (SEQ ID NO: 20).

[0031] 1.3 Construction of the Gene Encoding the HuEPO-L-vFc_(γ1) Fusion Protein

[0032] The hinge domain of human IgG1 heavy chain contains 15 amino acid residues (GluProLysSerCysAspLysThrHisThrCysProProCysPro, SEQ ID NO:24) including three cysteine residues. Out of these 3 cysteine residues, the 2nd and 3rd are involved in the formation of disulfide bonding between two heavy chains. The 1st cysteine residue is involved in the disulfide bonding to the light chain of IgG. Since there is no light chain present in the Fc fusion protein molecule, this cysteine residue may pair with other cysteine residues, leading to nonspecific disulfide bonding. The hinge domain of Fc_(γ1) can be truncated to eliminate the 1^(st) cysteine residue (AspLysThrHisThrCysProProCysPro, SEQ ID NO: 25). The gene encoding the Fc_(γ1) region is obtained by reverse transcription and PCR using RNA prepared from human leukocytes and appropriate 5′ primer (SEQ ID NO:13) and 3′ primer (SEQ ID NO:4). Resulting DNA fragments containing the truncated hinge and complete sequences of CH2 and CH3 domains of Fc_(γ1) is used as the template to generate the Fc_(γ1) variant with Leu234Val, Leu235Ala, and Pro331Ser mutations (vFc_(γ1)).

[0033] One way to incorporate these mutations is as follows: two segments are produced and then assembled by using the natural Fc_(γ1) as the template in overlapping PCR. The 5′ segment is generated by using SEQ ID NO:14 as the 5′ primer and SEQ ID NO:5 as the 3′ primer. This 5′ primer contains the Leu234Val, Leu235Ala mutations and the 3′ primer contains the Pro331Ser mutation. The 3′ segment is generated by using SEQ ID NO:6 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. These 5′ and 3′ segments are then joined at the region covering the Pro331Ser mutation by using SEQ ID NO:14 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. This amplified fragment of approximately 650 bp in length, together with a synthetic oligonucleotide of 55 bases (SEQ ID NO:15) containing Leu234Val and Leu235Ala, are joined in PCR by using SEQ ID NO:16 as the 5′ primer and SEQ ID NO:4 as the 3′ primer. The SEQ ID NO:16 primer contains sequences encoding a 16-amino acid Gly-Ser peptide linker including the BamHI site. The resulting DNA fragments of approximately 700 bp in length are inserted into a holding vector such as pUC19 at the BamHI and EcoRI sites to give the pL-vFcγ1 plasmid. The sequence of the gene is confirmed by DNA sequencing.

[0034] To prepare the HuEPO-L-vFc_(γ1) fusion gene, the EPO fragment is excised from the pEPO plasmid with HindIII and BamHI and inserted to the 5′-end of the peptide linker in the pL-vFcγ1 plasmid to give the pEPO-L-vFcγ1 plasmid. The fusion gene comprising HuEPO, a 16-amino acid Gly-Ser peptide linker, and the Fc_(γ1) variant gene is then inserted at the HindIII and EcoRI sites of a mammalian expression vector, such as pcDNA3 (Invitrogen), as described for the HuEPO-L-vFc_(γ2) fusion protein. The final expression vector plasmid is designated as pEFP1. FIG. 2C shows a fusion gene (SEQ ID NO: 21) containing sequences encoding HuEPO, a 16-amino acid peptide linker (GlySerGlyGlyGlySerGlyGlyGlyGlySerGlyGlyGlyGlySer, SEQ ID NO: 23), and the Fc_(γ1) variant with Leu234Val, Leu235Ala and Pro331Ser mutations, and its corresponding amino acid sequence (SEQ ID NO: 22).

[0035] 2. Expression of the Fusion Protein in Transfected Cell Lines

[0036] The recombinant pEFP1, pEFP2 or pEFP4 expression vector plasmid is transfected into a mammalian host cell line to achieve the expression of the HuEPO-L-vFc fusion protein. For stable high levels of expression, a preferred host cell line is Chinese Hamster Ovary (CHO) cells deficient in the DHFR enzyme (see, for example, U.S. Pat. No. 4,818,679). A preferred method of transfection is electroporation. Other methods, including calcium phosphate co-precipitation, lipofectin, and protoplast fusion, can also be used. For electroporation, 10 μg of plasmid DNA linearized with BspCI is added to 2 to 5×10⁷ cells in a cuvette using Gene Pulser Electroporator (Bio-Rad Laboratories, Hercules, Calif.) set at an electric field of 250 V and a capacitance of 960 μFd. Two days following the transfection, the media are replaced with growth media containing 0.8 mg/ml of G418. Transfectants resistant to the selection drug are tested for the secretion of the fusion protein by anti-human IgG Fc ELISA. Quantitation of the expressed fusion protein can also be carried out by ELISA using anti-HuEPO assays. The wells producing high levels of the Fc fusion protein are subcloned by limiting dilutions on 96-well tissue culture plates.

[0037] To achieve higher levels of the fusion protein expression, co-amplification is preferred by utilizing the gene of DHFR which can be inhibited by the MTX drug. In growth media containing increasing concentrations of MTX, the transfected fusion protein gene is co-amplified with the DHFR gene. Transfectants capable of growing in media with up to 1 μg/ml of MTX are again subcloned by limiting dilutions. The subcloned cell lines are further analyzed by measuring the secretion rates. Several cell lines yielding secretion rate levels over about 10 to about 30 μg/10⁶ cells/24 h, are adapted to suspension culture using serum-free growth media. The conditioned media are then used for the purification of the fusion protein. One cell line, E1-6, secreting HuEPO-L-vFc_(γ2) at about 20 μg/10⁶ cells/24 h was adapted to grow in suspension culture in 100-ml spinner flasks. After 10 days, cumulative concentration of the fusion protein reached approximately 200 μg/ml as analyzed by quantitative ELISA using the purified HuEPO-L-vFc_(γ2) protein as the standards. A single-cell subclone of E1-6, designated E1-6-A18, exhibited a secretion rate of HuEPO-L-vFc_(γ2) at about 38 μg/10⁶ cells/24 h.

[0038] Sugar side chain structures are crucial for the in vivo activity of EPO. The terminal sugar chain of the Asn-linked carbohydrate contains sialic acids, repeating poly-N-acetyllactosamine and glucose. Recombinant HuEPO expressed in certain mammalian cells such as NS0 is known to give proteins with low sialic acid content. Removal of sialic acids, which leads to exposure of the penultimate galactose residues, increases the affinity for hepatic asialoglycoprotein binding lectin. This trapping pathway results in decrease of in vivo biological activity as measured in whole animals. Recombinant HuEPO produced in CHO cells exhibit glycosylation patterns very similar to that found in the natural EPO (see, for example, Takeuchi et al., Proc. Natl. Acad. Sci. USA, 86:7819-22, 1989). The HuEPO-L-vFc fusion proteins expressed and produced in accordance with this invention will show similar or higher biological activities when compared to rHuEPO on a molar basis.

[0039] 3. Purification and Characterization of the Fusion Protein

[0040] Conditioned media containing the fusion protein are titrated with 1 N NaOH to a pH of 7 to 8 and filtered through a 0.45 micron cellulose nitrate filter. The filtrate is loaded onto a Prosep A column equilibrated in phosphate-buffered saline (PBS). After binding of the fusion protein to Prosep A, the flow-through fractions are discarded. The column is washed with PBS until OD at 280 nm is below 0.01. The bound fusion protein is then eluted with 0.1 M citrate buffer at pH 3.75. After neutralizing with 0.4 volume of 1 M K₂HPO₄, fractions containing purified protein are pooled and dialyzed against PBS. The solution is then filtered through a 0.22 micron cellulose nitrate filter and stored at 4° C. The purified HuEPO-L-vFc_(γ2) protein is quantitated by BCA protein assay using BSA as the standard. The molecular weight of the purified protein is in the range of 120 and 140 kDa by SDS-PAGE under non-reducing conditions. Under reducing conditions, the purified protein migrates around approximately 65 kDa. HPLC analysis did not show any appreciable aggregates in the purified HuEPO-L-vFc_(γ2) protein.

[0041] 4. In Vitro Biological Assays

[0042] Supernatants of transfectants or purified proteins can be tested for their ability to stimulate the proliferation of the 32D1.9 cells. The 32D1.9 cell line is a murine cell line that has been transfected with the human EPO-Rc gene. Like TF-1 cells (Kitamura et al., J. Cell. Physiol., 140:323-334, 1989), the 32D1.9 cells express human EPO-Rc on their cell surface and are responsive to EPO. The cells are maintained in growth medium (RPMI-1640 medium containing 10% FCS and human IL-5 at 1 to 5 ng/ml). Log phase 32D1.9 cells are collected and washed with assay medium (growth medium without human IL-5). A total of 1×10⁴ cells per sample of 32D1.9 in 50 μl is added to each well of a 96-well tissue culture plate. The cells are incubated with 50 μl of assay media containing various concentrations of the HuEPO-L-vFc fusion protein or the rHuEPO control from 0.001 to 30 nM each. The plate is kept at 37° C. and 5% CO₂ in a humidified incubator for 4 days before 10 μl of Methylthiazolylphenyl-tetrazolium bromide (MTT, 2.5 mg/ml in PBS) is added to each well. After 4 h, the cells and formazan are solubilized by adding 100 μl per well of 10% SDS in 0.01 N HCl. The plate is then read at 550 nm with the reference beam set at 690 nm. The OD reading is plotted against the concentration of rHuEPO or the fusion protein. The inflection point of the sigmoidal curve represents the concentration at which 50% of the maximal effect, ED50, is induced. The biological activity of HuEPO-L-vFc relative to that of rHuEPO can therefore be compared quantitatively. FIG. 3 shows the in vitro biological activity of the HuEPO-L-vFc_(γ2) fusion protein. Varying concentrations of rHuEPO or HuEPO-L-vFc_(γ2) were assayed for their abilities to stimulate the proliferation of the 32D1.9 cells. On a molar basis, HuEPO-L-vFc_(γ2) exhibits similar activity as that of rHuEPO, about 3 to about 4×10⁶ units/μmole.

[0043] Supernatants of transfectants or purified proteins can also be tested for their ability to stimulate the proliferation and differentiation of human bone marrow progenitor cells to form red blood cell colonies, colony forming unit-erythroid (CFU-E). The procedure is as follows. Light-density cells from human bone marrow centrifuged over Ficoll-Pague are washed and resuspended at 1×10⁶ cells/ml in Iscove's modified Dulbecco's medium (IMDM) containing 5% FCS. These cells are incubated in a tissue culture dish overnight at 37° C. and 5% CO₂ to remove all adherent cells including monocytes, macrophages, endothelial cell, and fibroblasts. Cells in suspension are then adjusted to 1×10⁵ cells/ml in IMDM containing 5% FCS. For the assay, 0.3 ml of cells, 15 μl of stem cell factor at 20 μg/ml, 2.4 ml of methylcellulose, and 0.3 ml of media containing several concentrations of HuEPO-L-vFc (or rHuEPO control) are mixed. One ml of this cell mixture is plated on a 35-mm petri dish. The dishes are then kept at 37° C. and 5% CO₂ for 10 to 14 d before the colonies are counted. A dose responsive curve can be plotted against the concentrations of HuEPO-L-vFc relative to those of rHuEPO.

[0044] 5. In Vivo Pharmacokinetic Studies in Rats

[0045] 5.1 Experimental Protocol

[0046] One hundred male SD rats with an average body weight of about 200 g each were checked by retro-orbital bleeds for red blood cell counts (RBC) and hemoglobin values (Hb). Fifty-seven rats within a small range of RBC were put into 19 groups with 3 rats in each group. One group was designated as the control group to receive i.v. injection of 0.25 ml of PBS per kg weight. Remaining 18 groups received i.v. injection of HuEPO-L-vFc_(γ2) at 100 μg/kg. Blood samples were taken at different time points after injection: 1 min, 2.5 min, 5 min, 10 min, 20 min, 1 h, 2 h, 4 h, 1 d, 2 d, 3 d, 4 d, 5 d, 6 d, 7 d, 8 d, 9 d, and 10 d. One ml each of whole blood from days 2 to 10 was used to test for RBC and Hb. At all time points, 2 ml of whole blood was collected into tubes containing anticoagulant, cells were removed, and plasma was frozen at −70° C. until assay was carried out.

[0047] 5.2 The Effect of i. v. Injection of HuEPO-L-vFc_(γ2) on RBC

[0048] In Table 2, starting day 5 after i. v. injection of HuEPO-L-vFc_(γ2), RBC showed a significant increase when compared to that in the control group (P<0.1). FIG. 4 is a graph using data from Table 2. TABLE 2 Effect of i.v. injection of HuEPO-L-vFc_(γ2) on RBC (10⁶ cells/μl; ±s, n = 3) Group Before injection After injection Difference Control 6.11 ± 0.10 6.88 ± 0.60 0.78 ± 0.52  48 h 6.10 ± 0.06 7.05 ± 0.30 0.95 ± 0.34  72 h 6.10 ± 0.16 6.59 ± 0.23 0.49 ± 0.38  96 h 6.09 ± 0.04 7.18 ± 0.54 1.09 ± 0.58 120 h 6.04 ± 0.15 7.66 ± 0.30 1.62 ± 0.22* 144 h 6.10 ± 0.16 7.35 ± 0.16 1.25 ± 0.02 168 h 6.07 ± 0.22 7.47 ± 0.06 1.40 ± 0.27 192 h 6.09 ± 0.14 7.84 ± 0.06 1.75 ± 0.09** 216 h 6.09 ± 0.10 7.59 ± 0.08 1.50 ± 0.08* 240 h 6.08 ± 0.17 8.00 ± 0.26 1.92 ± 0.10**

[0049] 5.3 The Effect of i. v. Injection of HuEPO-L-vFc_(γ2) on Hb

[0050] In Table 3, starting 72 h after i. v. injection of HuEPO-L-vFc_(γ2), Hb showed a significant increase when compared to that in the control group (P<0.1). FIG. 5 is a graph using data from Table 3. TABLE 3 Effect of i.v. injection of HuEPO-L-vFc_(γ2) on Hb (g/L; ±s, n = 3) Group Before injection After injection Difference Control 140 ± 2.52 151 ± 8.9 10.7 ± 8.4  48 h 134 ± 2.09 158 ± 6.4 24.0 ± 8.2  72 h 143 ± 10 158 ± 3.5 15.0 ± 13.2*  96 h 139 ± 1.5 167 ± 12.1 28.3 ± 10.7* 120 h 141 ± 5.7 176 ± 4.2 35.0 ± 5.0** 144 h 139 ± 2.7 166 ± 1.2 27.3 ± 2.5** 168 h 140 ± 2.3 170 ± 8.9 29.7 ± 6.7** 192 h 141 ± 3.1 183 ± 1.2 42.0 ± 2.0*** 216 h 139 ± 3.6 172 ± 2.5 32.7 ± 4.0** 240 h 139 ± 7.2 185 ± 3.8 45.3 ± 6.0***

[0051] 5.4 Serum Levels of HuEPO-L-vFc_(γ2) and Pharmacokinetics

[0052] Concentrations of HuEPO-L-vFc_(γ2) in serum samples collected at different time points were analyzed quantitatively by ELISA using anti-HuEPO and anti-human IgG Fc antibodies. Results are shown in Table 4 and also as a graph in FIG. 6. Data were also analyzed using the 3P87 software to generate the pharmacokinetic parameters in Table 5. Different time points gave the best fit using a two-compartment model with weight on 1/c. From this calculation, the T_(1/2)α and T_(1/2)β of HuEPO-L-vFc_(γ2) in rats are 0.63 h and 14.2 h, respectively. The plasma clearance of HuEPO in rats has also been reported to be nonlinear, often with an initial rapid distribution, followed by a slower elimination phase. The half-life of the β elimination phase has been reported to be about 2 to 3 h (see, for example, Spivak et al., Blood, 73:90-99, 1989; Fukuda et al., Blood, 73:84-89, 1989; Kinoshita et al., Arzneimitteljorschung, 42:174-178, 1992). The results of this study indicates that the elimination half-life of HuEPO-L-vFc_(γ2) is much longer than that of rHuEPO, indicating the prolonged presence of the fusion protein in rats. TABLE 4 Serum concentrations of HuEPO-L-vFc_(γ2) after injection Serum Concentration (ng/ml) Time after Corrected injection Rat 1 Rat 2 Rat 3 Average ± SE Value   1 Min. 2989 2654 2282 2641.7 ± 353.7 2624.5 2.5 Min. 2866 2614 2170 2550.0 ± 352.4 2532.9   5 Min. 2866 2747 2974 2862.3 ± 113.5 2845.2  10 Min. 2790 2635 2419 2614.7 ± 186.3 2597.5  20 Min. 2549 2384 2798 2577.0 ± 208.4 2559.9   1 Hr. 1244 1545 1694 1494.3 ± 229.2 1477.2   2 Hr. 1149 1067 1134 1116.7 ± 43.7  1099.5   4 Hr. 1108 1054 1182 1114.7 ± 64.3  1097.5   1 Day 250 240 288 259.3 ± 25.3 242.2   2 Days 103 170 100 124.3 ± 39.6 107.2   3 Days 98 44 109  83.7 ± 34.8 66.5   4 Days 56 44 63 54.3 ± 9.6 37.2   5 Days 24 26 16 22.0 ± 5.3 4.9   6 Days 30 33 35 32.7 ± 2.5 15.5   7 Days 16 17 15 16.0 ± 1.0 −1.1   8 Days 19 20 19.5 ± 0.7 2.4   9 Days 19 13 16.0 ± 4.2 −1.1  10 Days 22 14 15 17.0 ± 4.4 −0.1

[0053] TABLE 5 Pharmacokinetic parameters after injection of HuEPO-L-vFc_(γ2) Parameter unit Value Standard Error (S.E.) A ng/ml 1743.15 220.29 α 1/hr 1.1019 0.3856 B ng/ml 1072.20 207.27 β 1/hr 0.04894 0.00829 V(c) (μg/kg)/(ng/ml) 0.00710 t_(1/2α) hr 0.6291 t_(1/2β) hr 14.1632 AUC ng · hr/ml 23490.4 CL(s) μg/kg/hr/(ng/ml) 0.0085

[0054] The examples described above are for illustration purposes only. They are not intended and should not be interpreted to limit either the scope or the spirit of this invention. It can be appreciated by those skilled in the art that many other variations or substitutes can be used as equivalents for the purposes of this invention, which is defined solely by the written description and the following claims.

1 28 1 29 DNA Artificial Sequence misc_feature (1)..(29) synthetic 1 cccaagcttg gcgcggagat gggggtgca 29 2 27 DNA Artificial sequence misc_feature (1)..(27) synthetic 2 cggatccgtc ccctgtcctg caggcct 27 3 20 DNA Artificial Sequence misc_feature (1)..(20) synthetic 3 gagcgcaaat gttgtgtcga 20 4 28 DNA Artificial Sequence misc_feature (1)..(28) synthetic 4 ggaattctca tttacccgga gacaggga 28 5 29 DNA Artificial Sequence misc_feature (1)..(29) synthetic 5 tggttttctc gatggaggct gggaggcct 29 6 29 DNA Artificial Sequence misc_feature (1)..(29) Synthetic 6 aggcctccca gcctccatcg agaaaacca 29 7 69 DNA Artificial Sequence misc_feature (1)..(69) synthetic 7 cggatccggt ggcggttccg gtggaggcgg aagcggcggt ggaggatcag agcgcaaatg 60 ttgtgtcga 69 8 21 DNA Artificial Sequence misc_feature Synthetic 8 gagtccaaat atggtccccc a 21 9 28 DNA Artificial Sequence misc_feature (1)..(28) Synthetic 9 ggaattctca tttacccaga gacaggga 28 10 21 DNA Artificial Sequence misc_feature (1)..(21) Synthetic 10 cctgagttcg cggggggacc a 21 11 60 DNA Artificial Sequence misc_feature (1)..(60) Synthetic 11 gagtccaaat atggtccccc atgcccacca tgcccagcac ctgagttcgc ggggggacca 60 12 70 DNA Artificial Sequence misc_feature (1)..(70) Synthetic 12 cggatccggt ggcggttccg gtggaggcgg aagcggcggt ggaggatcag agtccaaata 60 tggtccccca 70 13 21 DNA Artificial Sequence misc_feature (1)..(21) Synthetic 13 gacaaaactc acacatgccc a 21 14 23 DNA Artificial Sequence misc_feature (1)..(23) Synthetic 14 acctgaagtc gcggggggac cgt 23 15 55 DNA Artificial Sequence misc_feature (1)..(55) Synthetic 15 gacaaaactc acacatgccc accgtgccca gcacctgaag tcgcgggggg accgt 55 16 70 DNA Artificial Sequence misc_feature (1)..(70) Synthetic 16 cggatccggt ggcggttccg gtggaggcgg aagcggcggt ggaggatcag acaaaactca 60 cacatgccca 70 17 1332 DNA Artificial Sequence HuEPO-L-vFc gamma2 (Figure 2A) 17 aagcttggcg cggagatggg ggtgcacgaa tgtcctgcct ggctgtggct tctcctgtcc 60 ctgctgtcgc tccctctggg cctcccagtc ctgggcgccc caccacgcct catctgtgac 120 agccgagtcc tggagaggta cctcttggag gccaaggagg ccgagaatat cacgacgggc 180 tgtgctgaac actgcagctt gaatgagaat atcactgtcc cagacaccaa agttaatttc 240 tatgcctgga agaggatgga ggtcgggcag caggccgtag aagtctggca gggcctggcc 300 ctgctgtcgg aagctgtcct gcggggccag gccctgttgg tcaactcttc ccagccgtgg 360 gagcccctgc agctgcatgt ggataaagcc gtcagtggcc ttcgcagcct caccactctg 420 cttcgggctc tgggagccca gaaggaagcc atctcccctc cagatgcggc ctcagctgct 480 ccactccgaa caatcactgc tgacactttc cgcaaactct tccgagtcta ctccaatttc 540 ctccggggaa agctgaagct gtacacaggg gaggcctgca ggacagggga cggatccggt 600 ggcggttccg gtggaggcgg aagcggcggt ggaggatcag agcgcaaatg ttgtgtcgag 660 tgcccaccgt gcccagcacc acctgtggca ggaccgtcag tcttcctctt ccccccaaaa 720 cccaaggaca ccctcatgat ctcccggacc cctgaggtca cgtgcgtggt ggtggacgtg 780 agccacgaag accccgaggt ccagttcaac tggtacgtgg acggcgtgga ggtgcataat 840 gccaagacaa agccacggga ggagcagttc aacagcacgt tccgtgtggt cagcgtcctc 900 accgttgtgc accaggactg gctgaacggc aaggagtaca agtgcaaggt ctccaacaaa 960 ggcctcccag cctccatcga gaaaaccatc tccaaaacca aagggcagcc ccgagaacca 1020 caggtgtaca ccctgccccc atcccgggag gagatgacca agaaccaggt cagcctgacc 1080 tgcctggtca aaggcttcta ccccagcgac atcgccgtgg agtgggagag caatgggcag 1140 ccggagaaca actacaagac cacacctccc atgctggact ccgacggctc cttcttcctc 1200 tacagcaagc tcaccgtgga caagagcagg tggcagcagg ggaacgtctt ctcatgctcc 1260 gtgatgcatg aggctctgca caaccactac acgcagaaga gcctctccct gtctccgggt 1320 aaatgagaat tc 1332 18 436 PRT Artificial Sequence HuEPO-L-vFc gamma2 with a 27-amino acid leader peptide (Figure 2A) 18 Met Gly Val His Glu Cys Pro Ala Trp Leu Trp Leu Leu Leu Ser Leu 1 5 10 15 Leu Ser Leu Pro Leu Gly Leu Pro Val Leu Gly Ala Pro Pro Arg Leu 20 25 30 Ile Cys Asp Ser Arg Val Leu Glu Arg Tyr Leu Leu Glu Ala Lys Glu 35 40 45 Ala Glu Asn Ile Thr Thr Gly Cys Ala Glu His Cys Ser Leu Asn Glu 50 55 60 Asn Ile Thr Val Pro Asp Thr Lys Val Asn Phe Tyr Ala Trp Lys Arg 65 70 75 80 Met Glu Val Gly Gln Gln Ala Val Glu Val Trp Gln Gly Leu Ala Leu 85 90 95 Leu Ser Glu Ala Val Leu Arg Gly Gln Ala Leu Leu Val Asn Ser Ser 100 105 110 Gln Pro Trp Glu Pro Leu Gln Leu His Val Asp Lys Ala Val Ser Gly 115 120 125 Leu Arg Ser Leu Thr Thr Leu Leu Arg Ala Leu Gly Ala Gln Lys Glu 130 135 140 Ala Ile Ser Pro Pro Asp Ala Ala Ser Ala Ala Pro Leu Arg Thr Ile 145 150 155 160 Thr Ala Asp Thr Phe Arg Lys Leu Phe Arg Val Tyr Ser Asn Phe Leu 165 170 175 Arg Gly Lys Leu Lys Leu Tyr Thr Gly Glu Ala Cys Arg Thr Gly Asp 180 185 190 Gly Ser Gly Gly Gly Ser Gly Gly Gly Gly Ser Gly Gly Gly Gly Ser 195 200 205 Glu Arg Lys Cys Cys Val Glu Cys Pro Pro Cys Pro Ala Pro Pro Val 210 215 220 Ala Gly Pro Ser Val Phe Leu Phe Pro Pro Lys Pro Lys Asp Thr Leu 225 230 235 240 Met Ile Ser Arg Thr Pro Glu Val Thr Cys Val Val Val Asp Val Ser 245 250 255 His Glu Asp Pro Glu Val Gln Phe Asn Trp Tyr Val Asp Gly Val Glu 260 265 270 Val His Asn Ala Lys Thr Lys Pro Arg Glu Glu Gln Phe Asn Ser Thr 275 280 285 Phe Arg Val Val Ser Val Leu Thr Val Val His Gln Asp Trp Leu Asn 290 295 300 Gly Lys Glu Tyr Lys Cys Lys Val Ser Asn Lys Gly Leu Pro Ala Ser 305 310 315 320 Ile Glu Lys Thr Ile Ser Lys Thr Lys Gly Gln Pro Arg Glu Pro Gln 325 330 335 Val Tyr Thr Leu Pro Pro Ser Arg Glu Glu Met Thr Lys Asn Gln Val 340 345 350 Ser Leu Thr Cys Leu Val Lys Gly Phe Tyr Pro Ser Asp Ile Ala Val 355 360 365 Glu Trp Glu Ser Asn Gly Gln Pro Glu Asn Asn Tyr Lys Thr Thr Pro 370 375 380 Pro Met Leu Asp Ser Asp Gly Ser Phe Phe Leu Tyr Ser Lys Leu Thr 385 390 395 400 Val Asp Lys Ser Arg Trp Gln Gln Gly Asn Val Phe Ser Cys Ser Val 405 410 415 Met His Glu Ala Leu His Asn His Tyr Thr Gln Lys Ser Leu Ser Leu 420 425 430 Ser Pro Gly Lys 435 19 1335 DNA Artificial Sequence HuEPO-L-vFc gamma4 (Figure 2B) 19 aagcttggcg cggagatggg ggtgcacgaa tgtcctgcct ggctgtggct tctcctgtcc 60 ctgctgtcgc tccctctggg cctcccagtc ctgggcgccc caccacgcct catctgtgac 120 agccgagtcc tggagaggta cctcttggag gccaaggagg ccgagaatat cacgacgggc 180 tgtgctgaac actgcagctt gaatgagaat atcactgtcc cagacaccaa agttaatttc 240 tatgcctgga agaggatgga ggtcgggcag caggccgtag aagtctggca gggcctggcc 300 ctgctgtcgg aagctgtcct gcggggccag gccctgttgg tcaactcttc ccagccgtgg 360 gagcccctgc agctgcatgt ggataaagcc gtcagtggcc ttcgcagcct caccactctg 420 cttcgggctc tgggagccca gaaggaagcc atctcccctc cagatgcggc ctcagctgct 480 ccactccgaa caatcactgc tgacactttc cgcaaactct tccgagtcta ctccaatttc 540 ctccggggaa agctgaagct gtacacaggg gaggcctgca ggacagggga cggatccggt 600 ggcggttccg gtggaggcgg aagcggcggt ggaggatcag agtccaaata tggtccccca 660 tgcccaccat gcccagcacc tgagttcgcg gggggaccat cagtcttcct gttcccccca 720 aaacccaagg acactctcat gatctcccgg acccctgagg tcacgtgcgt ggtggtggac 780 gtgagccagg aagaccccga ggtccagttc aactggtacg tggatggcgt ggaggtgcat 840 aatgccaaga caaagccgcg ggaggagcag ttcaacagca cgtaccgtgt ggtcagcgtc 900 ctcaccgtcc tgcaccagga ctggctgaac ggcaaggagt acaagtgcaa ggtctccaac 960 aaaggcctcc cgtcctccat cgagaaaacc atctccaaag ccaaagggca gccccgagag 1020 ccacaggtgt acaccctgcc cccatcccag gaggagatga ccaagaacca ggtcagcctg 1080 acctgcctgg tcaaaggctt ctaccccagc gacatcgccg tggagtggga gagcaatggg 1140 cagccggaga acaactacaa gaccacgcct cccgtgctgg actccgacgg ctccttcttc 1200 ctctacagca ggctaaccgt ggacaagagc aggtggcagg aggggaatgt cttctcatgc 1260 tccgtgatgc atgaggctct gcacaaccac tacacacaga agagcctctc cctgtctctg 1320 ggtaaatgag aattc 1335 20 437 PRT Artificial Sequence HuEPO-L-vFc gamma4 with a 27-amino acid leader peptide (Figure 2B) 20 Met Gly Val His Glu Cys Pro Ala Trp Leu Trp Leu Leu Leu Ser Leu 1 5 10 15 Leu Ser Leu Pro Leu Gly Leu Pro Val Leu Gly Ala Pro Pro Arg Leu 20 25 30 Ile Cys Asp Ser Arg Val Leu Glu Arg Tyr Leu Leu Glu Ala Lys Glu 35 40 45 Ala Glu Asn Ile Thr Thr Gly Cys Ala Glu His Cys Ser Leu Asn Glu 50 55 60 Asn Ile Thr Val Pro Asp Thr Lys Val Asn Phe Tyr Ala Trp Lys Arg 65 70 75 80 Met Glu Val Gly Gln Gln Ala Val Glu Val Trp Gln Gly Leu Ala Leu 85 90 95 Leu Ser Glu Ala Val Leu Arg Gly Gln Ala Leu Leu Val Asn Ser Ser 100 105 110 Gln Pro Trp Glu Pro Leu Gln Leu His Val Asp Lys Ala Val Ser Gly 115 120 125 Leu Arg Ser Leu Thr Thr Leu Leu Arg Ala Leu Gly Ala Gln Lys Glu 130 135 140 Ala Ile Ser Pro Pro Asp Ala Ala Ser Ala Ala Pro Leu Arg Thr Ile 145 150 155 160 Thr Ala Asp Thr Phe Arg Lys Leu Phe Arg Val Tyr Ser Asn Phe Leu 165 170 175 Arg Gly Lys Leu Lys Leu Tyr Thr Gly Glu Ala Cys Arg Thr Gly Asp 180 185 190 Gly Ser Gly Gly Gly Ser Gly Gly Gly Gly Ser Gly Gly Gly Gly Ser 195 200 205 Glu Ser Lys Tyr Gly Pro Pro Cys Pro Pro Cys Pro Ala Pro Glu Phe 210 215 220 Ala Gly Gly Pro Ser Val Phe Leu Phe Pro Pro Lys Pro Lys Asp Thr 225 230 235 240 Leu Met Ile Ser Arg Thr Pro Glu Val Thr Cys Val Val Val Asp Val 245 250 255 Ser Gln Glu Asp Pro Glu Val Gln Phe Asn Trp Tyr Val Asp Gly Val 260 265 270 Glu Val His Asn Ala Lys Thr Lys Pro Arg Glu Glu Gln Phe Asn Ser 275 280 285 Thr Tyr Arg Val Val Ser Val Leu Thr Val Leu His Gln Asp Trp Leu 290 295 300 Asn Gly Lys Glu Tyr Lys Cys Lys Val Ser Asn Lys Gly Leu Pro Ser 305 310 315 320 Ser Ile Glu Lys Thr Ile Ser Lys Ala Lys Gly Gln Pro Arg Glu Pro 325 330 335 Gln Val Tyr Thr Leu Pro Pro Ser Gln Glu Glu Met Thr Lys Asn Gln 340 345 350 Val Ser Leu Thr Cys Leu Val Lys Gly Phe Tyr Pro Ser Asp Ile Ala 355 360 365 Val Glu Trp Glu Ser Asn Gly Gln Pro Glu Asn Asn Tyr Lys Thr Thr 370 375 380 Pro Pro Val Leu Asp Ser Asp Gly Ser Phe Phe Leu Tyr Ser Arg Leu 385 390 395 400 Thr Val Asp Lys Ser Arg Trp Gln Glu Gly Asn Val Phe Ser Cys Ser 405 410 415 Val Met His Glu Ala Leu His Asn His Tyr Thr Gln Lys Ser Leu Ser 420 425 430 Leu Ser Leu Gly Lys 435 21 1329 DNA Artificial Sequence HuEPO-L-vFc gamma1 (Figure 2C) 21 aagcttggcg cggagatggg ggtgcacgaa tgtcctgcct ggctgtggct tctcctgtcc 60 ctgctgtcgc tccctctggg cctcccagtc ctgggcgccc caccacgcct catctgtgac 120 agccgagtcc tggagaggta cctcttggag gccaaggagg ccgagaatat cacgacgggc 180 tgtgctgaac actgcagctt gaatgagaat atcactgtcc cagacaccaa agttaatttc 240 tatgcctgga agaggatgga ggtcgggcag caggccgtag aagtctggca gggcctggcc 300 ctgctgtcgg aagctgtcct gcggggccag gccctgttgg tcaactcttc ccagccgtgg 360 gagcccctgc agctgcatgt ggataaagcc gtcagtggcc ttcgcagcct caccactctg 420 cttcgggctc tgggagccca gaaggaagcc atctcccctc cagatgcggc ctcagctgct 480 ccactccgaa caatcactgc tgacactttc cgcaaactct tccgagtcta ctccaatttc 540 ctccggggaa agctgaagct gtacacaggg gaggcctgca ggacagggga cggatccggt 600 ggcggttccg gtggaggcgg aagcggcggt ggaggatcag acaaaactca cacatgccca 660 ccgtgcccag cacctgaagt cgcgggggga ccgtcagtct tcctcttccc cccaaaaccc 720 aaggacaccc tcatgatctc ccggacacct gaggtcacat gcgtggtggt ggacgtgagc 780 cacgaagacc ctgaggtcaa gttcaactgg tacgtggacg gcgtggaggt gcataatgcc 840 aagacaaagc cgcgggagga gcagtacaac agcacgtacc gggtggtcag cgtcctcacc 900 gtcctgcacc aggactggct gaatggcaag gagtacaagt gcaaggtctc caacaaagcc 960 ctcccagcct ccatcgagaa aaccatctcc aaagccaaag ggcagccccg agaaccacag 1020 gtgtacaccc tgcccccatc ccgggatgag ctgaccaaga accaggtcag cctgacctgc 1080 ctggtcaaag gcttctatcc cagcgacatc gccgtggagt gggagagcaa tgggcagccg 1140 gagaacaact acaagaccac gcctcccgtg ctggactccg acggctcctt cttcctctac 1200 agcaagctca ccgtggacaa gagcaggtgg cagcagggga acgtcttctc atgctccgtg 1260 atgcatgagg ctctgcacaa ccactacacg cagaagagcc tctccctgtc tccgggtaaa 1320 tgagaattc 1329 22 435 PRT Artificial Sequence HuEPO-L-vFc gamma1 with a 27-amino acid leader peptide (Figure 2C) 22 Met Gly Val His Glu Cys Pro Ala Trp Leu Trp Leu Leu Leu Ser Leu 1 5 10 15 Leu Ser Leu Pro Leu Gly Leu Pro Val Leu Gly Ala Pro Pro Arg Leu 20 25 30 Ile Cys Asp Ser Arg Val Leu Glu Arg Tyr Leu Leu Glu Ala Lys Glu 35 40 45 Ala Glu Asn Ile Thr Thr Gly Cys Ala Glu His Cys Ser Leu Asn Glu 50 55 60 Asn Ile Thr Val Pro Asp Thr Lys Val Asn Phe Tyr Ala Trp Lys Arg 65 70 75 80 Met Glu Val Gly Gln Gln Ala Val Glu Val Trp Gln Gly Leu Ala Leu 85 90 95 Leu Ser Glu Ala Val Leu Arg Gly Gln Ala Leu Leu Val Asn Ser Ser 100 105 110 Gln Pro Trp Glu Pro Leu Gln Leu His Val Asp Lys Ala Val Ser Gly 115 120 125 Leu Arg Ser Leu Thr Thr Leu Leu Arg Ala Leu Gly Ala Gln Lys Glu 130 135 140 Ala Ile Ser Pro Pro Asp Ala Ala Ser Ala Ala Pro Leu Arg Thr Ile 145 150 155 160 Thr Ala Asp Thr Phe Arg Lys Leu Phe Arg Val Tyr Ser Asn Phe Leu 165 170 175 Arg Gly Lys Leu Lys Leu Tyr Thr Gly Glu Ala Cys Arg Thr Gly Asp 180 185 190 Gly Ser Gly Gly Gly Ser Gly Gly Gly Gly Ser Gly Gly Gly Gly Ser 195 200 205 Asp Lys Thr His Thr Cys Pro Pro Cys Pro Ala Pro Glu Val Ala Gly 210 215 220 Gly Pro Ser Val Phe Leu Phe Pro Pro Lys Pro Lys Asp Thr Leu Met 225 230 235 240 Ile Ser Arg Thr Pro Glu Val Thr Cys Val Val Val Asp Val Ser His 245 250 255 Glu Asp Pro Glu Val Lys Phe Asn Trp Tyr Val Asp Gly Val Glu Val 260 265 270 His Asn Ala Lys Thr Lys Pro Arg Glu Glu Gln Tyr Asn Ser Thr Tyr 275 280 285 Arg Val Val Ser Val Leu Thr Val Leu His Gln Asp Trp Leu Asn Gly 290 295 300 Lys Glu Tyr Lys Cys Lys Val Ser Asn Lys Ala Leu Pro Ala Ser Ile 305 310 315 320 Glu Lys Thr Ile Ser Lys Ala Lys Gly Gln Pro Arg Glu Pro Gln Val 325 330 335 Tyr Thr Leu Pro Pro Ser Arg Asp Glu Leu Thr Lys Asn Gln Val Ser 340 345 350 Leu Thr Cys Leu Val Lys Gly Phe Tyr Pro Ser Asp Ile Ala Val Glu 355 360 365 Trp Glu Ser Asn Gly Gln Pro Glu Asn Asn Tyr Lys Thr Thr Pro Pro 370 375 380 Val Leu Asp Ser Asp Gly Ser Phe Phe Leu Tyr Ser Lys Leu Thr Val 385 390 395 400 Asp Lys Ser Arg Trp Gln Gln Gly Asn Val Phe Ser Cys Ser Val Met 405 410 415 His Glu Ala Leu His Asn His Tyr Thr Gln Lys Ser Leu Ser Leu Ser 420 425 430 Pro Gly Lys 435 23 16 PRT Artificial sequence Synthetic linker 23 Gly Ser Gly Gly Gly Ser Gly Gly Gly Gly Ser Gly Gly Gly Gly Ser 1 5 10 15 24 15 PRT Homo sapiens 24 Glu Pro Lys Ser Cys Asp Lys Thr His Thr Cys Pro Pro Cys Pro 1 5 10 15 25 10 PRT Homo sapiens 25 Asp Lys Thr His Thr Cys Pro Pro Cys Pro 1 5 10 26 232 PRT Homo sapiens 26 Glu Pro Lys Ser Cys Asp Lys Thr His Thr Cys Pro Pro Cys Pro Ala 1 5 10 15 Pro Glu Leu Leu Gly Gly Pro Ser Val Phe Leu Phe Pro Pro Lys Pro 20 25 30 Lys Asp Thr Leu Met Ile Ser Arg Thr Pro Glu Val Thr Cys Val Val 35 40 45 Val Asp Val Ser His Glu Asp Pro Glu Val Lys Phe Asn Trp Tyr Val 50 55 60 Asp Gly Val Glu Val His Asn Ala Lys Thr Lys Pro Arg Glu Glu Gln 65 70 75 80 Tyr Asn Ser Thr Tyr Arg Val Val Ser Val Leu Thr Val Leu His Gln 85 90 95 Asp Trp Leu Asn Gly Lys Glu Tyr Lys Cys Lys Val Ser Asn Lys Ala 100 105 110 Leu Pro Ala Pro Ile Glu Lys Thr Ile Ser Lys Ala Lys Gly Gln Pro 115 120 125 Arg Glu Pro Gln Val Tyr Thr Leu Pro Pro Ser Arg Asp Glu Leu Thr 130 135 140 Lys Asn Gln Val Ser Leu Thr Cys Leu Val Lys Gly Phe Tyr Pro Ser 145 150 155 160 Asp Ile Ala Val Glu Trp Glu Ser Asn Gly Gln Pro Glu Asn Asn Tyr 165 170 175 Lys Thr Thr Pro Pro Val Leu Asp Ser Asp Gly Ser Phe Phe Leu Tyr 180 185 190 Ser Lys Leu Thr Val Asp Lys Ser Arg Trp Gln Gln Gly Asn Val Phe 195 200 205 Ser Cys Ser Val Met His Glu Ala Leu His Asn His Tyr Thr Gln Lys 210 215 220 Ser Leu Ser Leu Ser Pro Gly Lys 225 230 27 228 PRT Homo sapiens 27 Glu Arg Lys Cys Cys Val Glu Cys Pro Pro Cys Pro Ala Pro Pro Val 1 5 10 15 Ala Gly Pro Ser Val Phe Leu Phe Pro Pro Lys Pro Lys Asp Thr Leu 20 25 30 Met Ile Ser Arg Thr Pro Glu Val Thr Cys Val Val Val Asp Val Ser 35 40 45 His Glu Asp Pro Glu Val Gln Phe Asn Trp Tyr Val Asp Gly Val Glu 50 55 60 Val His Asn Ala Lys Thr Lys Pro Arg Glu Glu Gln Phe Asn Ser Thr 65 70 75 80 Phe Arg Val Val Ser Val Leu Thr Val Val His Gln Asp Trp Leu Asn 85 90 95 Gly Lys Glu Tyr Lys Cys Lys Val Ser Asn Lys Gly Leu Pro Ala Pro 100 105 110 Ile Glu Lys Thr Ile Ser Lys Thr Lys Gly Gln Pro Arg Glu Pro Gln 115 120 125 Val Tyr Thr Leu Pro Pro Ser Arg Glu Glu Met Thr Lys Asn Gln Val 130 135 140 Ser Leu Thr Cys Leu Val Lys Gly Phe Tyr Pro Ser Asp Ile Ala Val 145 150 155 160 Glu Trp Glu Ser Asn Gly Gln Pro Glu Asn Asn Tyr Lys Thr Thr Pro 165 170 175 Pro Met Leu Asp Ser Asp Gly Ser Phe Phe Leu Tyr Ser Lys Leu Thr 180 185 190 Val Asp Lys Ser Arg Trp Gln Gln Gly Asn Val Phe Ser Cys Ser Val 195 200 205 Met His Glu Ala Leu His Asn His Tyr Thr Gln Lys Ser Leu Ser Leu 210 215 220 Ser Pro Gly Lys 225 28 229 PRT Homo sapiens 28 Glu Ser Lys Tyr Gly Pro Pro Cys Pro Ser Cys Pro Ala Pro Glu Phe 1 5 10 15 Leu Gly Gly Pro Ser Val Phe Leu Phe Pro Pro Lys Pro Lys Asp Thr 20 25 30 Leu Met Ile Ser Arg Thr Pro Glu Val Thr Cys Val Val Val Asp Val 35 40 45 Ser Gln Glu Asp Pro Glu Val Gln Phe Asn Trp Tyr Val Asp Gly Val 50 55 60 Glu Val His Asn Ala Lys Thr Lys Pro Arg Glu Glu Gln Phe Asn Ser 65 70 75 80 Thr Tyr Arg Val Val Ser Val Leu Thr Val Leu His Gln Asp Trp Leu 85 90 95 Asn Gly Lys Glu Tyr Lys Cys Lys Val Ser Asn Lys Gly Leu Pro Ser 100 105 110 Ser Ile Glu Lys Thr Ile Ser Lys Ala Lys Gly Gln Pro Arg Glu Pro 115 120 125 Gln Val Tyr Thr Leu Pro Pro Ser Gln Glu Glu Met Thr Lys Asn Gln 130 135 140 Val Ser Leu Thr Cys Leu Val Lys Gly Phe Tyr Pro Ser Asp Ile Ala 145 150 155 160 Val Glu Trp Glu Ser Asn Gly Gln Pro Glu Asn Asn Tyr Lys Thr Thr 165 170 175 Pro Pro Val Leu Asp Ser Asp Gly Ser Phe Phe Leu Tyr Ser Arg Leu 180 185 190 Thr Val Asp Lys Ser Arg Trp Gln Glu Gly Asn Val Phe Ser Cys Ser 195 200 205 Val Met His Glu Ala Leu His Asn His Tyr Thr Gln Lys Ser Leu Ser 210 215 220 Leu Ser Leu Gly Lys 225 

1. A recombinant HuEPO-L-vFc fusion protein comprising HuEPO, a peptide linker, and a human IgG Fc variant.
 2. The peptide linker in claim 1 containing about 20 or fewer amino acids is present between HuEPO and the human IgG Fc variant; and the peptide linker comprises two or more amino acids selected from the group consisting of glycine, serine, alanine, and threonine.
 3. The human IgG Fc variant in claim 1 or claim 2 comprising a hinge, CH2, and CH3 domains of human IgG2 with Pro331Ser mutation as SEQ ID NO:
 18. 4. The human IgG Fc variant in claim 1 or claim 2 comprising a hinge, CH2, and CH3 domains of human IgG4 with Ser228Pro and Leu235Ala mutations as SEQ ID NO:
 20. 5. The human IgG Fc variant in claim 1 or claim 2 comprising a hinge, CH2, and CH3 domains of human IgB1 with Leu234Val, Leu235Ala, and Pro331Ser mutations as SEQ ID NO:
 22. 6. The HuEPO-L-vFc fusion protein of any of the preceding claims exhibits in vitro biological activity similar to or higher than that of rHuEPO on a molar basis.
 7. A CHO-derived cell line producing the HuEPO-L-vFc fusion protein of any of the preceding claims in its growth medium in excess of 10 μg per million cells in a 24 hour period.
 8. The CHO-derived cell line producing the HuEPO-L-vFc fusion protein of claim 7 in its growth medium in excess of 30 μg per million cells in a 24 hour period.
 9. The CHO-derived cell line producing the HuEPO-L-vFc fusion protein of claim 1, wherein the human IgG Fc variant comprises a hinge, CH2, and CH3 domains of human IgG selected from the group consisting of IgB1 as SEQ ID NO: 22, IgG2 as SEQ ID NO: 18, and IgG4 as SEQ ID NO: 20, the IgG Fc contains amino acid mutations to attenuate effector functions, a flexible peptide linker containing about 20 or fewer amino acids is present between HuEPO and human IgG Fc variant, and the HuEPO-L-vFc fusion protein exhibits in vitro biological activity similar to or higher than that of rHuEPO on a molar basis.
 10. A method for making a recombinant fusion protein comprising HuEPO, a flexible peptide linker, and a human IgG Fc variant, which method comprises: (a) generating a CHO-derived cell line; (b) growing the cell line under conditions the recombinant protein is expressed in its growth medium in excess of 10 μg per million cells in a 24 hour period; and (c) purifying the expressed protein from step (b), wherein the recombinant fusion protein exhibits in vitro biological activity similar to or higher than that of rHuEPO on a molar basis.
 11. The method of claim 10, wherein the flexible peptide linker containing about 20 or fewer amino acids is present between HuEPO and the human IgG Fc variant; and the peptide linker comprises two or more amino acids selected from the group consisting of glycine, serine, alanine, and threonine.
 12. The method of claim 10, wherein the human IgG Fc variant comprises a hinge, CH2, and CH3 domains of human IgG2 with Pro331Ser mutation.
 13. The method of claim 10, wherein the human IgG Fc variant comprises a hinge, CH2, and CH3 domains of human IgG4 with Ser228Pro and Leu235Ala mutations.
 14. The method of claim 10, wherein the human IgG Fc variant comprises a hinge, CH2, and CH3 domains of human IgG1 with Leu234Val, Leu235Ala, and Pro331Ser mutations as SEQ ID NO:
 18. 15. The method of any claim of claims 10, 11, 12, 13, and 14, wherein step (b) is in excess of 30 μg per million cells in a 24 hour period.
 16. The method of claim 10, wherein the human IgG Fc variant comprises a hinge, CH2, and CH3 domains of human IgG4 with Ser228Pro and Leu235Ala mutations as SEQ ID NO:
 20. 17. The method of claim 16, wherein step (b) is in excess of 30 μg per million cells in a 24 hour period.
 18. The method of claim 10, wherein the human IgG Fc variant comprises a hinge, CH2, and CH3 domains of human IgB1 with Leu234Val, Leu235Ala, and Pro331Ser mutations as SEQ ID NO:
 22. 19. The method of claim 18, wherein step (b) is in excess of 30 μg per million cells in a 24 hour period.
 20. A method for making a recombinant fusion protein comprising HuEPO, a flexible peptide linker, and a human IgG Fc variant, which method comprises: (a) generating a CHO-derived cell line; (b) growing the cell line under conditions the recombinant protein is expressed in its growth medium in excess of 10 μg per million cells in a 24 hour period; and (c) purifying the expressed protein from step (b), wherein the recombinant fusion protein exhibits in vitro biological activity similar to or higher than that of rHuEPO on a molar basis; wherein the flexible peptide linker containing about 20 or fewer amino acids is present between HuEPO and the human IgG Fc variant; and the peptide linker comprises two or more amino acids selected from the group consisting of glycine, serine, alanine, and threonine; wherein the human IgG Fc variant comprises a hinge, CH2, and CH3 domains selected from the group consisting of human IgG2 with Pro331Ser mutation as SEQ ID NO: 18, human IgG4 with Ser228Pro and Leu235Ala mutations as SEQ ID NO: 20, and human IgB1 with Leu234Val, Leu235Ala, and Pro331Ser mutations as SEQ ID NO:
 22. 